With regard to fluid bed combustion (FBC) plants, it is desired to reheat steam in the Rankine cycle without compromising the constraints imposed by the fluid bed combustion process. This is because the constraints of the combustion process can supersede the desired features of the reheat steam system, thus attaining a less than optimum reheat cycle and a less favorable plant design.
In the past, external heat exchangers using recycled bed material have been used to supply heat to the reheat circuit. However, when the recycle energy of this bed material was not high enough, other means, such as directly utilizing the in-furnace tube surface, became necessary. Unfortunately, the low tube mass of such a reheat circuit causes problems as a result of the very high heat input of the FBC process.
Should the absorbed heat be too great, reheat spray attemperation can be used to temper the fluctuation of the reheat steam. Unfortunately, such spray attemperation is very inefficient. Furthermore, it is desired for the control range of such reheated steam to be as wide as possible under a variety of load conditions (down to about 50% load or less).
One constraint in the arrangement of a reheat circuit in a fossil fired steam generator is the fact that the pressure drop must be small. The efficiency of the Rankine steam cycle is significantly reduced if large pressure losses occur. To minimize such losses, it is common to install a large number of steam flow paths in the furnace which are short in length and which have few bend or other restriction losses. Additionally, the tube flow area or diameter must be large enough to keep the mass flow velocity low thereby reducing friction and shock loss pressure drops. Furthermore, the distribution of steam flow into each of these numerous tubes cannot be accompanied by a high flow control pressure drop. In most cases, reheat pressure is relatively low (about 600 psi) while its volume of flow is relatively large (about 90% of main steam flow).
It is thus an object of this invention to provide a reheat circuit for generating hot reheat steam whose temperature can be controlled over a wide range of operating parameters. Another object of this invention is to provide a means of reheating that separately utilizes both main steam and flue gas as a heat source. Still another object of this invention is to provide for such reheating while remaining within the constraints of the FBC process and without introducing large pressure losses. Yet another object of this invention is to provide a means for attemperation should the reheat temperature become excessive. These and other objects and advantages of this invention will become obvious upon further investigation.